Join between tables in two different databases?

In MySQL, I have two different databases -- let's call them A and B.

Is it possible to perform a join between a table that is in database A, to a table that is in database B?

Yes, assuming the account has appropriate permissions you can use:

SELECT <...>
FROM A.table1 t1 JOIN B.table2 t2 ON t2.column2 = t1.column1;

You just need to prefix the table reference with the name of the database it resides in.

SELECT <...> 
FROM A.tableA JOIN B.tableB

SELECT *
FROM A.tableA JOIN B.tableB

SELECT *
  FROM A.tableA JOIN B.tableB
  ON A.tableA.id = B.tableB.a_id;

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